The office, which underwent a deep clean on Monday following positive tests, has closed as a precautionary measure.
Mick Dunlavey, the treasurer at the Communication Workers’ Union (CWU), said: “This morning (Wednesday) on the grounds of safety the Barnsley delivery office will be closed. Cudworth and Grimethorpe will also close as a precautionary measure until full and comprehensive checks have been made.
“There will be disruptions to services but the safety of our people comes first.”